Ferrocenophanium Stability and Catalysis

Ferrocenium catalysis is a vibrant research area, and an increasing number of ferrocenium-catalyzed processes have been reported in the recent years. However, the ferrocenium cation is not very stable in solution, which may potentially hamper catalytic applications. In an effort to stabilize ferrocenium-type architectures by inserting a bridge between the cyclopentadienyl rings, we investigated two ferrocenophanium (or ansa-ferrocenium) cations with respect to their stability and catalytic activity in propargylic substitution reactions. One of the ferrocenophanium complexes was characterized by single crystal X-ray diffraction. Cyclic voltammetry experiments of the ferrocenophane parent compounds were performed in the absence and presence of alcohol nucleophiles, and the stability of the cations in solution was judged based on the reversibility of the electron transfer. The experiments revealed a moderate stabilizing effect of the bridge, albeit the effect is not very pronounced or straightforward. Catalytic propargylic substitution test reactions revealed decreased activity of the ferrocenophanium cations compared to the ferrocenium cation. It appears that the somewhat stabilized ferrocenophanium cations show decreased catalytic activity.

However, ferrocenium cations are not very stable in solution, especially in chlori ated solvents and under aerobic conditions [68][69][70][71][72][73]. The ferrocenium cation decompos in solution, and this fact has also been taken advantage of in Diels-Alder reactions [73 Catalyst decomposition is a big concern, as it may counter efforts to design catalysts f specific purposes. As such, efforts to investigate catalyst decomposition and the design more robust catalyst systems are important issues.
Herein, we investigate ferrocenophanium cations as potentially more stable com plexes compared to ferrocenium and test them as catalysts in propargylic substitution r actions. Ferrocenophanes (or ansa-ferrocenes) are ferrocene complexes, where the two c clopentadienyl (Cp) ring systems are bridged (e.g., 1 and 2 in Scheme 1, top). We hypot esized that a bridge between the Cp rings stabilizes the complex, and would slow dow or block decomposition pathways, such as decomposition by Cp ring loss or intermole ular decomposition by nucleophilic attack on the iron (which would in a ferrocenopha be somewhat protected by the bridge). We determined the stability of ferrocenophaniu cations in the absence and presence of nucleophiles by cyclic voltammetry, and we a plied the cations in propargylic substitution test reactions (Scheme 1, bottom). Scheme 1. Ferrocene and ferrocenophanium complexes and catalytic application.

Results and Discussion
Ferrocenophanes or ansa-ferrocenes are a well-known compound class [74]. Th bridges can be all-carbon [74,75] or contain heteroatoms [76,77], and several synthe methods for their access have been described [78]. Either the two ring systems are co nected by a series of organic transformations, or a preformed, bidentate dicyclopentad enyl ligand is coordinated to an iron center ("flytrap method") [79]. We set out to synth size and investigate the known, mono-bridged "ketoferrocenophane" complex 1, whi can be obtained from ferrocene in an acylation-Michael reaction sequence (Scheme

Results and Discussion
Ferrocenophanes or ansa-ferrocenes are a well-known compound class [74]. The bridges can be all-carbon [74,75] or contain heteroatoms [76,77], and several synthetic methods for their access have been described [78]. Either the two ring systems are connected by a series of organic transformations, or a preformed, bidentate dicyclopentadienyl ligand is coordinated to an iron center ("flytrap method") [79]. We set out to synthesize and investigate the known, mono-bridged "ketoferrocenophane" complex 1, which can be obtained from ferrocene in an acylation-Michael reaction sequence (Scheme 1) [80,81]. The keto functionality can be reduced to obtain the hydrocarbon-only bridge in the "ferrocenophane" 2 (Scheme 1) [80,81]. The chemistry to obtain ferrocenophanes 1 and 2 worked as described in the literature. The conversion from 1 to 2 can be followed by IR, as the C=O stretch in 1 disappears after reduction. However, the workup procedures are tedious, and the yields are generally not very high, as also indicated in the original synthetic procedures. Oxidation of the two complexes led to the corresponding ferrocenophanium salts 1 + SbF 6 [82] and 2 + SbF 6 [83,84]. The oxidation of 1 and 2 can be observed by UV-vis spectroscopy, as 1 + SbF 6 and 2 + SbF 6 give a new band around 620 nm, as it is characteristic of ferrocenophanium cations [83]. NMR investigation of the ferrocenophanium salts is, due to their paramagnetic nature, not possible. Consequently, our investigations were plagued by difficulties accessing the ferrocenophanium ions in large quantities.
The molecular structure of 1 + SbF 6 determined by single crystal X-ray diffraction analysis is shown in Figure 1. The details of intensity data collection and refinement are given in the Supporting Information. The crystal structure refinement parameters are summarized in Table 1.  [80,81]. The keto functionality can be reduced to obtain the hydrocarbon-only bridge in the "ferrocenophane" 2 (Scheme 1) [80,81]. The chemistry to obtain ferrocenophanes 1 and 2 worked as described in the literature. The conversion from 1 to 2 can be followed by IR, as the C=O stretch in 1 disappears after reduction. However, the workup procedures are tedious, and the yields are generally not very high, as also indicated in the original synthetic procedures. Oxidation of the two complexes led to the corresponding ferrocenophanium salts 1 + SbF6 [82] and 2 + SbF6 [83,84]. The oxidation of 1 and 2 can be observed by UV-vis spectroscopy, as 1 + SbF6 and 2 + SbF6 give a new band around 620 nm, as it is characteristic of ferrocenophanium cations [83]. NMR investigation of the ferrocenophanium salts is, due to their paramagnetic nature, not possible. Consequently, our investigations were plagued by difficulties accessing the ferrocenophanium ions in large quantities. The molecular structure of 1 + SbF6 -determined by single crystal X-ray diffraction analysis is shown in Figure 1. The details of intensity data collection and refinement are given in the Supporting Information. The crystal structure refinement parameters are summarized in Table 1. The complex 1 + SbF6 crystallizes in monoclinic spacegroup P21/c with one ferrocenophanium complex and one SbF6 -anion in the asymmetric unit. From the crystal structure it is observed that the monosubstituted C5H4 rings are η 5 coordinated to the central iron atom with C(Cp)-Fe bond lengths ranging from 2.020(6) Å to 2.127(8) Å, showing a significant range. The short C(Cp)-Fe bond length values such as C1(Cp)-Fe1 = 2.020(6) Å and C6(Cp)-Fe1 = 2.040(9) Å is attributed to the substitutions at C1 and C6 positions, respectively. The metallocene conformation is eclipsed with a slightly distorted ferrocene framework showing a Cp(centroid)-Fe-Cp(centroid) angle of 171.8(2)˚ and Cp(centroid)-Fe distance of 1.692(5) Å and 1.6939(13) Å respectively. The carbonyl group is trigonally planar with the C1-C11-C12 [117.4(6)°] angle being slightly smaller than C1-C11-O1 [121.2(9)°] and O1-C11-C12 [120.9(9)°], respectively. Interestingly, the trigonal planar carbonyl group is oriented out of the adjacent Cp-planes with calculated dihedral angles of 41.56(3)° and 51.44(3)°, respectively. In the SbF6 -anions, the bond lengths and angles are within the expected range with Sb-F distances ranging from 1.771(11) Å to 1.878(5) Å. In the three-dimensional crystal structure, the molecules are mainly stabilized by C-H…O The complex 1 + SbF 6 crystallizes in monoclinic spacegroup P21/c with one ferrocenophanium complex and one SbF 6 anion in the asymmetric unit. From the crystal structure it is observed that the monosubstituted C 5 H 4 rings are η 5 coordinated to the central iron atom with C(Cp)-Fe bond lengths ranging from 2.020(6) Å to 2.  Table S1). The oxygen atom O1 acts as a bifurcated acceptor forming two C-H . . . O interactions such as C10-H10. . . O1 and C12-H12A. . . O1 which connects the adjacent ferrocenophanium complexes (Supplementary Materials, Figure S1). Adjacent ferrocenophanium chains are further bridged by SbF 6 anions via three C-H. . . F interactions (C3-H3. . . F3, C5-H5. . . F1, and C7-H7. . . F3), which results in the formation of a three-dimensional network in the crystalline solid. The interlinkage of the ferrocenophanium complex chain and the SbF 6 anions via C-H. . . F interaction is illustrated in Figure S2.
1537.43 (7)  1.12→nd −1.08 The bridge in the ferrocenophanium cation 1 + causes the Cp rings to be inclined to each other with a dihedral (or tilt) angle of 14.39(6) • (α in Figure 2) [85,86]. Therefore, there is some open space on the opposite side of the bridge, where we hypothesized catalysis can take place. The tilt is smaller compared to ferrocenophanes with bridges consisting of two atoms, where angles around 22 • have been reported [87]. Ferrocenophanes with a bridge consisting of only one atom exhibit angles between 27 and 38 • , putting a substantial strain on the system [79,88]. Obviously, the shorter the bridge, the larger the tilt. In general, atoms directly bonded to the Cp ligands are located in the plane of the ring system. In ferrocenophanes, these atoms can be bent out of the plane of the Cp ring system and quantified with the dip angle [89]. The dip angle is defined as the deviation of the angle between the centroid of the Cp ring, the ipso-carbon atom, and the first carbon atom of the bridge from the ideal 180 • (β in Figure 2) [86,89]. The angles were calculated to be 2.81 • (C13) and 9.44 • (C11, the C=O carbon). It appears that the C1 carbon of the Cp ring bearing the C=O unit has a slightly higher sp 3 character. quantified with the dip angle [89]. The dip angle is defined as the devia between the centroid of the Cp ring, the ipso-carbon atom, and the firs the bridge from the ideal 180° (β in Figure 2) [86,89]. The angles were calc (C13) and 9.44° (C11, the C=O carbon). It appears that the C1 carbon of the the C=O unit has a slightly higher sp 3 character.

Cyclic Voltammetry Experiments
We next investigated the stability of the ferrocenophanium cations by cyclic voltammetry (CV) [90,91]. Related experiments have been performed with other ferrocenophanes [92,93]. In a typical CV experiment, the ferrocenophanes are oxidized to the corresponding ferrocenophanium cations. The reversibility of this process, as indicated by the ratio of the i c /i a current, would give insight into the stability of 1 + and 2 + , compared to the ferrocenium cation. A fully reversible process would give an i c /i a ratio of 1. As shown in Scheme 1 (bottom), the catalytic propargylic substitution test reaction involves the propargylic alcohol 3 and n-butanol (4) as the nucleophile. We also investigated the stability of 1 + and 2 + in the presence of alcohol nucleophiles to investigate whether they are part of potential decomposition chemistry. An i c /i a ratio smaller than 1 may indicate an electrochemical oxidation followed by an irreversible chemical reaction (EC mechanism) [91].
Representative CV traces of ferrocene, 1 and 2 in CH 2 Cl 2 are shown in Figure 3, in the absence (top) and presence of n-BuOH (bottom). As can be seen, all three compounds give the typical "duck-shaped" voltammograms expected for diffusion-controlled, reversible electron-transfer processes [91]. Plots of the peak current i p versus the square root of the scan rate give linear graphs for diffusion-controlled redox processes [91], which we observed (representative graphs are given in the Supporting Information). The numerical results of the CV experiments are compiled in Tables 2 and 3. In Table 2, CV data for ferrocene, and the ferrocenophane complexes 1 and 2 are compiled. This data allows for the comparison of the stability of the ferrocenium cation Fc + and the ferrocenophanium complexes 1 + and 2 + in solution in the absence of any other substrate. The ∆E values are a little higher than expected from theory for a reversible electron transfer (59 mV), which may be caused by the poorer conductivity of CH 2 Cl 2 and CH 3 CN solutions of n-Bu 4 NPF 6 utilized as the electrolyte; it may also indicate slow electrode kinetics [91]. The ∆E values are somewhat larger in CH 2 Cl 2 compared to CH 3 CN, which we ascribe to the lower conductivity of CH 2 Cl 2 , which leads to higher ohmic resistance of the solutions [91]. The complexes are barely soluble in other solvents. In Table 2, CV data for ferrocene, and the ferrocenophane complexes 1 and 2 are compiled. This data allows for the comparison of the stability of the ferrocenium cation Fc + and the ferrocenophanium complexes 1 + and 2 + in solution in the absence of any other substrate. The ΔE values are a little higher than expected from theory for a reversible electron transfer (59 mV), which may be caused by the poorer conductivity of CH2Cl2 and CH3CN solutions of n-Bu4NPF6 utilized as the electrolyte; it may also indicate slow electrode kinetics [91]. The ΔE values are somewhat larger in CH2Cl2 compared to CH3CN, which we ascribe to the lower conductivity of CH2Cl2, which leads to higher ohmic resistance of the solutions [91]. The complexes are barely soluble in other solvents. The E 1/2 values roughly correlate with the oxidation potential of metal complexes. For ferrocene, the standard redox potential is defined as +0.46 V in CH 2 Cl 2 and +0.40 V in CH 3 CN with n-Bu 4 NPF 6 as the electrolyte [90]. As derived from the E 1/2 values, the ketoferrocenophane 1 as a higher oxidation potential of 0.604 V and 0.658 V in CH 2 Cl 2 and CH 3 CN, respectively (Table 2, entries 2 and 5). These values are higher than that for ferrocene itself, which may be caused by the keto functionality in 1; it may withdraw electron density from the Cp ring system connected to it, resulting in a higher oxidation potential of the ketoferrocenophane complex 1. In turn, for ferrocenophane 2, the redox potentials in CH 2 Cl 2 and CH 3 CN (+0.318 V and +0.395 V, respectively; entries 3 and 6) are lower than for ferrocene. Here, the electron-donating alkyl bridge may overall increase the electron density of the Cp ring systems, resulting in a lower oxidation potential.
The i c /i a ratios allow for the assessment of the stability of the complexes in the solution. If the i c /i a ratio is smaller than 1, decomposition of the oxidized species may occur, so that there is less material left after oxidation to be reduced back to the starting material, resulting in a lower reduction current, following an EC mechanism described above [91].
As can be seen, the i c /i a ratios for the three complexes are fairly similar, ranging from 0.98 to 0.86 in any solvent. It seems that for the typical timeframe of the CV experiment (several seconds to minutes, depending on the sweep rate), only a small fraction of the oxidized ferrocene, ketoferrocenophanium 1 + , and ferrocenophanium 2 + decompose under the reaction conditions. This behavior may not be unexpected. Ferrocene is known to be fairly stable under CV conditions with fast sweep rates, and it appears that the establishment of a bridge between the Cp rings does not change that significantly during the short time frame of a CV experiment. However, we were interested to determine how the stability changes in the presence of the catalysis substrates. As mentioned, we applied the ferrocenium cation as a catalyst for propargylic substitution reactions (Scheme 1, bottom), where, besides the catalyst, the tertiary propargylic alcohol 3 and alcohol nucleophiles such as n-butanol (4), MeOH or i-PrOH were present [46,47]. Related CV experiments have been performed by others with ferrocenophanes in the presence of an imidazole base [94]. We performed the same CV experiments as in Table 2 in the presence of propargylic alcohol 3 and a number of primary and secondary aliphatic alcohols. We were interested in whether the stability of the ferrocenophanium complexes changes in the presence of alcohols, as reflected by the i c /i a ratios. The results are compiled in Table 3, representative CV traces are shown in Figure 3 (bottom) in the presence of n-BuOH. As can be seen, the duck shape is, in general, maintained in the presence of n-BuOH.
For ferrocene, the ∆E and the E 1/2 values are comparable to the measurements in the absence of alcohol. In the presence of MeOH, i-PrOH, and the propargylic alcohol 3, in CH 2 Cl 2 or CH 3 CN, the ferrocene seems to be fairly stable, as judged by the i c /i a ratios, which range from 0.78 to 0.96. In CH 3 CN as the solvent, there is a notable difference for n-BuOH as the nucleophile. In the presence of n-butanol, the i c /i a ratio dropped to 0.78 (entry 2). Interestingly, in the presence of MeOH, the i c /i a ratio stayed at around 0.94 (entry 1). The drop in stability in n-BuOH can be rationalized by a nucleophilic attack of the alcohol on the ferrocenium cation. It appears that MeOH would not undergo such an attack, which may be somewhat counterintuitive. We speculated that MeOH nucleophile is better solvated by the CH 3 CN solvent, though. In turn, in the presence of i-PrOH, the stability of the ferrocenium cation is also high, as reflected by an i c /i a ratio of 0.95 to 0.96 (entries 3 and 7). This may be a consequence of the low nucleophilicity of the secondary i-PrOH compared to the primary alcohol n-butanol.
Interestingly, the ketoferrocenophanium cation 1 + shows in CH 3 CN a lower, but consistent stability, as demonstrated by i c /i a ratios around 0.81 (entries 8 to 12). The ratios stay constant across different nucleophiles. Presumably, a chemical change of 1 + takes place without the nucleophile being directly involved. The stability for different alcohol nucleophiles is slightly higher in CH 2 Cl 2 (i c /i a ratios of 0.75 to 0.87 entries 13 to 16); here, the presence of the propargylic alcohol 3 leads to a slight decrease in stability, for which we do not have a clear explanation. Lower solvation of 3 may play a role here again.
However, the cation derived from the ferrocenophane 2 seems to be the most stable under these conditions, with average i c /i a ratios of 0.91 to 0.97 in CH 3 CN and CH 2 Cl 2 in the presence of n-BuOH and MeOH. It appears that the ferrocenophanium cation 2 + is somewhat more stable than the ferrocenophanium cation 1 + and the ferrocenium cation itself in the presence of MeOH and n-BuOH.
A possible explanation may be that the alkyl bridge in 2 protects the complex from bimolecular decomposition, assuming that the decomposition process involves the attack of n-BuOH on the iron center. The slightly more extended decomposition of the ferrocenium cation in the presence of n-BuOH may be due to the lack of the protecting bridge, facilitating the attack on the iron center. The ketoferrocenophanium complex 1 + seems overall to be somewhat less stable than the ferrocenophanium cation 2 + . Here, the electron-withdrawing keto group on the Cp ring may make the iron center more electrophilic, facilitating an attack of the alcohol on the iron center, despite some protection from the bridge. Also, involvement of the carbonyl group in the decomposition process, for example, through the formation of a hemiacetal, may play a role.
We also performed the experiments in Table 3 at different sweep rates, and for ferrocene and ferrocenophane 1 in presence of n-BuOH, the results are compiled in Table 4. Here, for ferrocene, a clear dependency of the sweep rate on the i c /i a ratios can be observed in CH 3 CN. The i c /i a ratios increase with increasing sweep rate. At a slower sweep rate, the ferrocenium ion is exposed for a longer time to the n-BuOH nucleophile, resulting in decomposition to a larger extent, which leads to lower i c /i a ratios. The i c /i a ratios for the ferrocenophanes 1 and 2 are relatively independent of the sweep rate with the exception of ketoferrocenophane 1, where the i c /i a ratio decreases with increasing sweep rate in CH 2 Cl 2 . It may be speculated that for 1 + and 2 + , decomposition by a nucleophilic attack is, within the timeframe of the CV experiment, diminished due to the protecting bridge in the ferrocenophanes. The results in Tables 2-4 are of interest in several respects. The bridges in the ferrocenophanes provide some protection for their respective cations in solution, and the protecting effect seems to be dependent on the solvent and the type of the nucleophile. However, the trend is less pronounced than expected. Ferrocene seems to be slightly more stable under the conditions in Table 3 than the ketoferrocenophane 1. It appears that the ferrocenium cation decomposes faster in the presence of n-BuOH, so the bridge in 1 may provide some protection over extended periods of time. In some instances, the decomposition increases with decreasing sweep rate. The findings may have some mechanistic impacts. We assume for the propargylic substitution reaction in Scheme 1 (bottom) an ionic mechanism, where first a propargylic carbocation is generated, which then can be attacked by the alcohol nucleophile [46]. The ferrocenium or ferrocenophanium cations to be applied for chemistry like in Scheme 1 (bottom) may undergo a chemical change substantially in the presence of n-butanol prior to or during catalysis.

Catalytic Experiments
In order to find out whether the catalyst stability has an impact on the activity, we employed the 1 + SbF 6 and 2 + SbF 6 in the propargylic test reaction in Scheme 1 (bottom) and two more reactions and compared them to the efficiency of the ferrocenium cation previously established in our research group. The results are compiled in Table 5. The reaction of the propargylic alcohol 3 with n-BuOH gave previously the propargylic ether product 5 [47]. The propargylic substitution reaction of the cyclopropyl-substituted propargylic alcohol 6 also resulted previously in the ring-opened eneyne product 7 when ferrocenium hexafluorophosphate (FcPF 6 ) was employed as a catalyst; the thiophenyl-substituted propargylic alcohol 8 gave the substitution product 9 with the cyclopropyl ring still intact [46].
Overall, the complexes 1 + SbF 6 and 2 + SbF 6 appear to be not as catalytically active as the ferrocenium cation itself. While some product was observed by GC, we were able to isolate the products in only 1 to 44% yields. The reactions in Table 5 are routine in our laboratory, and when employing FcPF 6 , we obtain frequently yields of at least 30% and higher [46,47]. Of the alcohols employed in Table 5, propargylic alcohol 3 showed the least activity in substitution reactions in our previous experiments [46,47]. Obviously, 1 + SbF 6 and 2 + SbF 6 seem not reactive enough to activate the propargylic alcohol 3 toward substitution. The cyclopropyl-substituted propargylic alcohol 6 is activated toward nucleophilic substitution [46], and both catalysts 1 + SbF 6 and 2 + SbF 6 result in moderate yields for the formation of eneyne 7. The thiophenyl-substituted propargylic alcohol 8 is also activated toward nucleophilic substitution [46]; however, both catalysts gave no detectable product 9 or only very little of it. The yield of 7 is slightly higher for 2 + ; however, the general trend points toward a lower catalytic activity. and 2 + SbF6 seem not reactive enough to activate the propargylic alcohol 3 toward substitution. The cyclopropyl-substituted propargylic alcohol 6 is activated toward nucleophilic substitution [46], and both catalysts 1 + SbF6 and 2 + SbF6 result in moderate yields for the formation of eneyne 7. The thiophenyl-substituted propargylic alcohol 8 is also activated toward nucleophilic substitution [46]; however, both catalysts gave no detectable product 9 or only very little of it. The yield of 7 is slightly higher for 2 + ; however, the general trend points toward a lower catalytic activity.  [47]. 3 Data taken from [46].
The low yields in Table 5 for the ferrocenophanium complexes may indicate that the somewhat increased stability of the complexes decreases their activity. Catalysts must exhibit a level of reactivity in order to show catalytic activity. Another point to be considered is the actual catalytically active species when FcPF6 is employed as a catalyst. While the and 2 + SbF6 seem not reactive enough to activate the propargylic alcohol 3 toward substitution. The cyclopropyl-substituted propargylic alcohol 6 is activated toward nucleophilic substitution [46], and both catalysts 1 + SbF6 and 2 + SbF6 result in moderate yields for the formation of eneyne 7. The thiophenyl-substituted propargylic alcohol 8 is also activated toward nucleophilic substitution [46]; however, both catalysts gave no detectable product 9 or only very little of it. The yield of 7 is slightly higher for 2 + ; however, the general trend points toward a lower catalytic activity.  [47]. 3 Data taken from [46].
The low yields in Table 5 for the ferrocenophanium complexes may indicate that the somewhat increased stability of the complexes decreases their activity. Catalysts must exhibit a level of reactivity in order to show catalytic activity. Another point to be considered is the actual catalytically active species when FcPF6 is employed as a catalyst. While the and 2 + SbF6 seem not reactive enough to activate the propargylic alcohol 3 toward substitution. The cyclopropyl-substituted propargylic alcohol 6 is activated toward nucleophilic substitution [46], and both catalysts 1 + SbF6 and 2 + SbF6 result in moderate yields for the formation of eneyne 7. The thiophenyl-substituted propargylic alcohol 8 is also activated toward nucleophilic substitution [46]; however, both catalysts gave no detectable product 9 or only very little of it. The yield of 7 is slightly higher for 2 + ; however, the general trend points toward a lower catalytic activity.  [47]. 3 Data taken from [46].
The low yields in Table 5 for the ferrocenophanium complexes may indicate that the somewhat increased stability of the complexes decreases their activity. Catalysts must exhibit a level of reactivity in order to show catalytic activity. Another point to be considered is the actual catalytically active species when FcPF6 is employed as a catalyst. While the 74% 2 0% 0% activity in substitution reactions in our previous experiments [46,47]. Obviously, 1 SbF6 and 2 + SbF6 seem not reactive enough to activate the propargylic alcohol 3 toward substitution. The cyclopropyl-substituted propargylic alcohol 6 is activated toward nucleophilic substitution [46], and both catalysts 1 + SbF6 and 2 + SbF6 result in moderate yields for the formation of eneyne 7. The thiophenyl-substituted propargylic alcohol 8 is also activated toward nucleophilic substitution [46]; however, both catalysts gave no detectable product 9 or only very little of it. The yield of 7 is slightly higher for 2 + ; however, the general trend points toward a lower catalytic activity.  [47]. 3 Data taken from [46].
The low yields in Table 5 for the ferrocenophanium complexes may indicate that the somewhat increased stability of the complexes decreases their activity. Catalysts must exhibit a level of reactivity in order to show catalytic activity. Another point to be considered is the actual catalytically active species when FcPF6 is employed as a catalyst. While the activity in substitution reactions in our previous experiments [46,47]. Obviously, 1 SbF6 and 2 + SbF6 seem not reactive enough to activate the propargylic alcohol 3 toward substitution. The cyclopropyl-substituted propargylic alcohol 6 is activated toward nucleophilic substitution [46], and both catalysts 1 + SbF6 and 2 + SbF6 result in moderate yields for the formation of eneyne 7. The thiophenyl-substituted propargylic alcohol 8 is also activated toward nucleophilic substitution [46]; however, both catalysts gave no detectable product 9 or only very little of it. The yield of 7 is slightly higher for 2 + ; however, the general trend points toward a lower catalytic activity.  [47]. 3 Data taken from [46].
The low yields in Table 5 for the ferrocenophanium complexes may indicate that the somewhat increased stability of the complexes decreases their activity. Catalysts must exhibit a level of reactivity in order to show catalytic activity. Another point to be considered is the actual catalytically active species when FcPF6 is employed as a catalyst. While the 37% 3 13% 44% activity in substitution reactions in our previous experiments [46,47]. Obviously, 1 + SbF6 and 2 + SbF6 seem not reactive enough to activate the propargylic alcohol 3 toward substitution. The cyclopropyl-substituted propargylic alcohol 6 is activated toward nucleophilic substitution [46], and both catalysts 1 + SbF6 and 2 + SbF6 result in moderate yields for the formation of eneyne 7. The thiophenyl-substituted propargylic alcohol 8 is also activated toward nucleophilic substitution [46]; however, both catalysts gave no detectable product 9 or only very little of it. The yield of 7 is slightly higher for 2 + ; however, the general trend points toward a lower catalytic activity.  [47]. 3 Data taken from [46].
The low yields in Table 5 for the ferrocenophanium complexes may indicate that the somewhat increased stability of the complexes decreases their activity. Catalysts must exhibit a level of reactivity in order to show catalytic activity. Another point to be considered is the actual catalytically active species when FcPF6 is employed as a catalyst. While the activity in substitution reactions in our previous experiments [46,47]. Obviously, 1 + SbF6 and 2 + SbF6 seem not reactive enough to activate the propargylic alcohol 3 toward substitution. The cyclopropyl-substituted propargylic alcohol 6 is activated toward nucleophilic substitution [46], and both catalysts 1 + SbF6 and 2 + SbF6 result in moderate yields for the formation of eneyne 7. The thiophenyl-substituted propargylic alcohol 8 is also activated toward nucleophilic substitution [46]; however, both catalysts gave no detectable product 9 or only very little of it. The yield of 7 is slightly higher for 2 + ; however, the general trend points toward a lower catalytic activity. 1 Reaction conditions: propargylic alcohol and alcohol in an equimolar amount, CH2Cl2 solvent at 45 °C for 2 h followed by chromatographic workup; 2 Data taken from [47]. 3 Data taken from [46].
The low yields in Table 5 for the ferrocenophanium complexes may indicate that the somewhat increased stability of the complexes decreases their activity. Catalysts must exhibit a level of reactivity in order to show catalytic activity. Another point to be considered is the actual catalytically active species when FcPF6 is employed as a catalyst. While the 44% 3 -6% 1 Reaction conditions: propargylic alcohol and alcohol in an equimolar amount, CH 2 Cl 2 solvent at 45 • C for 2 h followed by chromatographic workup; 2 Data taken from [47]. 3 Data taken from [46].
The low yields in Table 5 for the ferrocenophanium complexes may indicate that the somewhat increased stability of the complexes decreases their activity. Catalysts must exhibit a level of reactivity in order to show catalytic activity. Another point to be considered is the actual catalytically active species when FcPF 6 is employed as a catalyst. While the ferrocenium cation may show some stability under cyclic voltammetry conditions (room temperature and short reaction time), it may undergo chemical changes under the catalytic conditions in Table 5 (45 • C and at least 2 h reaction time). Longer reaction times with ketoferrocenophanium complex 1 + did not result in higher yields, though. Whether the decomposition products of the ferrocenium or ferrocenophanium cations are the actual catalytically active species is currently under investigation in our research group. If the ferrocenophanium complexes show somewhat increased stability in solution, as hypothesized, the reactivity may just be lower.

Conclusions
Overall, cyclic voltammetry data presented herein establish a moderate protecting effect of the bridge in ferrocenophanium cations in presence of alcohol nucleophiles compared to ferrocenium. These findings were established by CV i c /i a values and their dependency on the sweep rate. However, catalytic experiments revealed that the ferrocenophanium cations showed overall lower catalytic activity compared to the ferrocenium cation, as judged by lower isolated catalysis product yields. While decreased catalytic stability may result in decreased catalytic activity, very reactive catalysts in solution may also show a higher tendency to decompose. The catalytic activity landscape of ferrocenium and ferrocenophanium cations investigated herein may be more complex than it appears at first glance.